Heterogeneity of Photosystem I reaction centers in barley leaves as related to the donation from stromal reductants

The light-response curves of P700 oxidation and time-resolved kinetics of P700⁺ dark re-reduction were studied in barley leaves using absorbance changes at 820 nm. Leaves were exposed to 45 °C and treated with either diuron or diuron plus methyl viologen (MV) to prevent linear electron flow from PS II to PSI and ferredoxin-dependent cyclic electron flow around PSI. Under those conditions, P700⁺ could accept electrons solely from soluble stromal reductants. P700 was oxidized under weak far-red light in leaves treated with diuron plus MV, while identical illumination was nearly ineffective in diuron-treated leaves in the absence of MV. When heat-exposed leaves were briefly illuminated with strong far-red light, which completely oxidized P700, the kinetics of P700⁺ dark reduction was fitted by a single exponential term with half-time of about 40 ms. However, two first-order kinetic components of electron flow to P700⁺ (fast and slow) were found after prolonged leaf irradiation. The light-induced modulation of the kinetics of P700⁺ dark reduction was reversed following dark adaptation. The fast component (half time of 80–90 ms) was 1.5 larger than the slow one (half time of about 1 s). No kinetic competition occurred between two pathways of electron donation to P700⁺ from stromal reductants. This suggests the presence of two different populations of PSI. This revised version was published online in June 2006 with corrections to the Cover Date.     

Induction kinetics of photosystem I-activated P700 oxidation in plant leaves and their dependence on pre-energization

Absorbance changes ΔA ₈₁₀ were measured in pea (Pisum sativum L., cv. Premium) leaves to track redox transients of chlorophyll P700 during and after irradiation with far red (FR) light under various preillumination conditions in the absence and presence of inhibitors and protonophorous uncoupler of photosynthetic electron transport. It was shown that cyclic electron transport (CET) in chloroplasts of pea leaves operates at its highest rate after preillumination of leaves with white light and is strongly suppressed after preillumination with FR light. The FR light-induced suppression was partly released during prolonged dark adaptation. Upon FR illumination of dark-adapted leaves, the induction of CET was observed, during which CET activity increased to the peak from the low level and then decreased gradually. The kinetics of P700 oxidation induced by FR light of various intensities in leaves preilluminated with white light were fit to empirical sigmoid curves containing two variables. In leaves treated with a protonophore FCCP, the amplitude of FR light-induced changes ΔA ₈₁₀ was strongly suppressed, indicating that the rate of CET is controlled by the pH gradient across the thylakoid membrane.     

Photoinduced changes in adsorption at 800 nm related to photosystem I functioning]

The spectra and kinetics of light-induced absorbance changes in the near-infrared region of subchloroplast fragments enriched by P700 were studied. An increase in absorbancy within the region of 725--900 nm upon illumination was characterized by a maximum around 810 nm and by "shoulders" around 760 and 870 nm. Similar effects of thermal inactivation and low temperatures on the duration of dark recovery of light-induced absorbance changes at 700 nm and within the region of 725--900 nm suggest that the absorbance changes in the near-infrared region are due to photooxidation of P700. The values of P700 differential extinction coefficients at 810 nm are 8,2.10(3) M-1.cm-1 for digitonin fragments and 7,7.10(3) M-1.cm-1 for fragments prepared with the use of diethyl ester. It was shown that the value of midpoint oxidation-reduction potential measured for the absorbance changes at 810 nm (+492 mv) is higher than that measured at 700 nm (+475 mv).     

Influence of electrochemical proton gradient on electron flow in photosystem I of pea leaves

Photoinduced changes in the redox state of photosystem I (PSI) primary donor, chlorophyll P700 were studied by measuring differential absorbance changes of pea leaves at 810 nm minus 870 nm (ΔA ₈₁₀). The kinetics of ΔA ₈₁₀ induced by 5-s pulses of white light were strongly affected by preillumination. In dark-adapted leaves, the light pulse caused a transient oxidation of P700 and its subsequent reduction. An identical pulse, applied after 30-s preillumination with white light, induced sequential appearance of two peaks of P700 oxidation. These kinetic differences of ΔA ₈₁₀ reflect regulatory changes of electron flow on the donor and acceptor sides of PSI induced by illumination of leaf for 20–40 s. The amplitude of ΔA ₈₁₀ second peak depended nonmonotonically on the dark interval preceding illumination: it increased with the length of dark period in the range 3–10 s and decreased upon longer dark intervals. The second wave of ΔA ₈₁₀ disappeared after the treatment with combination of ionophores preventing ΔpH and electric potential formation at the thylakoid membrane. In leaves treated with monensin eliminating ΔpH only, the ΔA ₈₁₀ signals become incompletely reversible and were characterized by slow relaxation in darkness. The results indicate an important role of electrochemical proton gradient in generation of the second wave of light-induced P700 oxidation.     

Photochemical activities and organization of photosynthetic apparatus of C3 and C4 plants grown under different light intensities

Changes in the photochemical activities, influenced by variation in the growth light intensity, were followed in typical C₃ (Phaseolus, Ipomoea) and C₄ (Amaranthus, Sorghum) plants. Progressive decrease in the growth light intensity accelerated the O-P fluorescence induction in whole leaves. Such acceleration of the fluorescence kinetics was found to be not due to enhanced photosystem II activity but possibly a result of reduced rate of electron flow between the two photosystems. This is supported by 4 lines of evidence: (1) by the Hill activity determined in the presence of electron acceptors functioning before and after plastoquinone; (2) the photosynthetic unit size determined after flash excitation showing variations that were apparently too small to account for the changes observed fluorescence induction; (3) modification of the kinetics of secondrange light-induced absorbance changes at 520 nm; and (4) absence of significant changes in the ratio of P₇₀₀/total chlorophyll ratio. The P₇₀₀/cytochrome f ratio, however, increased from the usual 1–1.5 to 3–4 in plants grown under 9% sunlight. Increase in the P₇₀₀/cytochrome f ratio was found to be due to a decrease in the cytochrome f/chlorophyll ratio, and this was due to perhaps to a simultaneous increase in chlorophyll and decrease in cytochrome content.     

Thermoluminescence and P700 redox kinetics as complementary tools to investigate the cyclic/chlororespiratory electron pathways in stress conditions in barley leaves

Cyclic electron flow around photosystem I drives additional proton pumping into the thylakoid lumen, which enhances the protective non-photochemical quenching and increases ATP synthesis. It involves several pathways activated independently. In whole barley leaves, P700 oxidation under far-red illumination and subsequent P700(+) dark reduction kinetics provide a major probe of the activation of cyclic pathways. Two 'intermediate' and 'slow' exponential reduction phases are always observed and they become faster after high light illumination, but dark inactivation of the Benson-Calvin cycle causes the emergence of both a transient in the P700 oxidation and a 'fast' phase in the P700(+) reduction. We investigate here the afterglow (AG) thermoluminescence emission as another tool to detect the activation of cyclic electron pathways from stroma reductants to the acceptor side of photosystem II. This transfer is activated by warming, yielding an AG band at about 45°C. However, treatments that accelerate the 'intermediate' and 'slow' P700(+) reduction phases (brief anoxia, hexose infiltration, fast dehydration of excised leaves) also produced a downshift of this AG band. This pathway ascribable to NADPH dehydrogenase (NDH) would be triggered by a deficit in ATP, while the 'fast' reduction phase corresponding to the ferredoxin plastoquinone reductase pathway is triggered by an overreduction of the photosystem I acceptor pool and is undetected in thermoluminescence. Contrastingly, slow dehydration of unwatered plants did not cause faster reduction of P700(+) nor temperature downshift of the AG band, that is no induction of the NDH pathway, whereas an increased intensity of the AG band indicated a strong NADPH + ATP assimilatory potential.     

Photochemical activities and organization of photosynthetic apparatus of C3 and C4 plants grown under different light intensities

Changes in the photochemical activities, influenced by variation in the growth light intensity, were followed in typical C₃ (Phaseolus, Ipomoea) and C₄ (Amaranthus, Sorghum) plants. Progressive decrease in the growth light intensity accelerated the O-P fluorescence induction in whole leaves. Such acceleration of the fluorescence kinetics was found to be not due to enhanced photosystem II activity but possibly a result of reduced rate of electron flow between the two photosystems. This is supported by 4 lines of evidence: (1) by the Hill activity determined in the presence of electron acceptors functioning before and after plastoquinone; (2) the photosynthetic unit size determined after flash excitation showing variations that were apparently too small to account for the changes observed fluorescence induction; (3) modification of the kinetics of second-range light-induced absorbance changes at 520 nm; and (4) absence of significant changes in the ratio of P₇₀₀/total chlorophyll ratio. The P₇₀₀/cytochrome f ratio, however, increased from the usual 1–1.5 to 3–4 in plants grown under 9% sunlight. Increase in the P₇₀₀/cytochrome f ratio was found to be due to a decrease in the cytochrome f/chlorophyll ratio, and this was due to perhaps to a simultaneous increase in chlorophyll and decrease in cytochrome content.     

Nonmonotonic Redox Conversions of P700 in Leaves Irradiated with Far-Red Light Originate from Variable Contributions of Several Alternative Electron Transport Pathways during the Induction Period

The origin of nonmonotonic changes in the redox state of P700, the primary electron donor of PSI, was investigated on predarkened barley (Hordeum vulgare L.) leaves exposed to far-red light. To accomplish this, the relaxation kinetics of absorbance changes at 830 nm, reflecting the dark reduction of P700⁺, were measured at different stages of the induction curve. The onset of far-red light resulted in rapid oxidation of P700, which was followed by its partial reduction and subsequent slow oxidation of P700 to a steady-state level. This steady-state level was usually attained within 10 s under far-red light. The relative contribution of the slow kinetic component of P700⁺ reduction decreased in parallel with the transient photoreduction of P700⁺ and increased upon a subsequent stage of P700 photooxidation. The contribution of the middle component to the dark reduction of P700⁺ increased monotonically with the length of far-red light irradiation. The relative amplitude of the fast component of P700⁺ reduction increased sharply during the first 3 s of irradiation and decreased upon longer light exposures. The rates of fast and slow components of dark reduction of P700⁺ remained constant upon illumination of dark-adapted leaves with far-red light for 1 s and longer periods. Thus, nonmonotonic changes in the redox state of P700 in barley leaves exposed to far-red light reflect variable contributions of few alternative electron transport pathways characterized by different rates of electron donation to PSI. The results show the principle possibility of switching-over between alternative pathways of PSI-related electron transfer within one complex of this photosystem. Such switching may occur irrespective of active operation or inhibition of ferredoxin-dependent electron transport.     

Effects of oxygen and photosynthesis carbon cycle reactions on kinetics of P700 redox transients in cyanobacterium <Emphasis Type="Italic">Arthrospira platensis</Emphasis> cells

Effects of oxygen and photosynthesis and respiration inhibitors on the electron transport in photosystem I (PSI) of the cyanobacterium Arthrospira platensis cells were studied. Redox transients of P700 were induced by illumination at 730 nm and monitored as kinetics of the absorption changes at 810 nm; to block electron influx from PSII, the measurements were performed in the presence of 30 microM 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Inhibitors of terminal oxidases (potassium cyanide and pentachlorophenol) insignificantly influenced the fast oxidation of P700 under aerobic conditions, whereas removal of oxygen significantly decelerated the accumulation of P700(+). In the absence of oxygen the slow oxidation of P700 observed on the first illumination was accelerated on each subsequent illumination, suggesting an activation of the carbon cycle enzymes. Under the same conditions, pentachlorophenol (an uncoupler) markedly accelerated the P700 photooxidation. Under anaerobic conditions, potassium cyanide (an inhibitor of carbon dioxide assimilation) failed to influence the kinetics of redox transients of P700, whereas iodoacetamide (an inhibitor of NADP(H)-glyceraldehyde-3-phosphate dehydrogenase) completely prevented the photooxidation of P700. Thus, the fast photooxidation of P700 in the A. platensis cells under aerobic conditions in the presence of DCMU was caused by electron transport from PSI onto oxygen, and complicated transient changes in the P700 photooxidation kinetics under anaerobic conditions (in the presence of DCMU) were due to involvement of NADP+ generated during the reducing phase of the carbon cycle.     

Effect of the Pool Size of Stromal Reductants on the Alternative Pathway of Electron Transfer to Photosystem I in Chloroplasts of Intact Leaves

The effect of elevated temperature on electron flow to plastoquinone pool and to PSI from sources alternative to PSII was studied in barley (Hordeum vulgare L.) and maize (Zea mays L.) leaves. Alternative electron flow was characterized by measuring variable fluorescence of chlorophyll and absorption changes at 830 nm that reflect redox changes of P700, the primary electron donor of PSI. The treatment of leaves with elevated temperature resulted in a transient increase in variable fluorescence after cessation of actinic light. This increase was absent in leaves treated with methyl viologen (MV). The kinetics of P700⁺ reduction in barley and maize leaves treated with DCMU and MV exhibited two exponential components. The rate of both components markedly increased with temperature of the heat pretreatment of leaves when the reduction of P700⁺ was measured after short (1 s) illumination of leaves. The acceleration of both kinetic components of P700⁺ reduction by high-temperature treatment was much less pronounced when P700⁺ reduction rate was measured after illumination of leaves for 1 min. Since the treatment of leaves with DCMU and MV inhibited both the electron flow to PSI from PSII and ferredoxin-dependent cycling of electrons around PSI, the accelerated reduction of P700⁺ indicated that high temperature treatment activated electron flow to PSII from reductants localized in the chloroplast stroma. We conclude that the lesser extent of activation of this process by elevated temperature after prolonged illumination of heat-inhibited leaves is caused by depletion of the pool stromal reductants in light due to photoinduced electron transfer from these reductants to oxygen.     

Characteristics of the leaf photosynthetic machinery in broad bean grown in zinc chloride aqueous solutions

The structural and functional characteristics of bean leaves (the content of chlorophyll, the rate of oxygen production, the slow fluorescence induction, and light-induced changes in the EPR signal I from oxidized reaction centers P700+) were investigated to obtain insight into the mechanism of influence of zinc chloride on the photosynthetic apparatus. Seedlings were grown on hydroponic medium containing ZnCl2 at concentrations from 10(-7) to 10(-3) M. At low concentrations of ZnCl2, a decrease in the content of chlorophyll per one unit of leaf mass was observed, while the rate of oxygen production per chlorophyll was increased. High concentrations of ZnCl2 in the hydroponic medium caused the slowed down the plant development and inhibited the light-induced production of oxygen. The changes in biophysical characteristics of leaves the parameter FM/FT of the slow fluorescence induction, and kinetics of redox transients of P700 induced by ZnCl2 were of similar character and correlated with the changes in photosynthetic activity. The data obtained demonstrate that structural and functional changes in the photosynthetic apparatus induced by the variations of growth conditions have adaptive character.     

Oxygen as an alternative electron acceptor in the photosynthetic electron transport chain of C3 plants

This study deals with effects of oxygen on the kinetics of P(700) photoinduced redox transitions and on induction transients of chlorophyll fluorescence in leaves of C(3) plants Hibiscus rosa-sinensis and Vicia faba. It is shown that the removal of oxygen from the leaf environment has a conspicuous effect on photosynthetic electron transport. Under anaerobic conditions, the concentration of oxidized P700 centers in continuous white light was substantially lower than under aerobic conditions. The deficiency of oxygen released non-photochemical quenching of chlorophyll fluorescence, thus indicating a decrease in the trans-thylakoid pH gradient (DeltapH). Quantitative analysis of experimental data within the framework of an original mathematical model has shown that the steady-state electron flux toward oxygen in Chinese hibiscus leaves makes up to approximately 40% of the total electron flow passing through photosystem 1 (PS1). The decrease in P700+ content under anaerobic conditions can be due to two causes: i) the retardation of electron outflow from PS1, and ii) the release of photosynthetic control (acceleration of electron flow from PS2 to P700+) owing to lower acidification of the intra-thylakoid space. At the same time, cyclic electron transport around PS1 was not stimulated in the oxygen-free medium, although such stimulation seemed likely in view of possible rearrangement of electron flows on the acceptor side of PS1. This conclusion stems from observations that the rates of P700+ reduction in DCMU-poisoned samples, both under aerobic and anaerobic conditions, were negligibly small compared to rates of electron flow from PS2 toward P700+ in untreated samples.     

Study of tobacco transformants to assess the role of chloroplastic NAD(P)H dehydrogenase in photoprotection of photosystems I and II

Nicotiana tabacum L. wild-type plants and transformants (DeltandhCKJ), deficient in functional NAD(P)H dehydrogenase (NDH), were subjected to high light at 20 degrees C and 4 degrees C for 2 h to examine a possible role of NDH-mediated cyclic electron flow in protecting photosystems I and II from photoinhibition. Photochemical activity of photosystem I (PSI) was assessed by means of P700 absorbance changes at 810 nm. In addition, potential photosystem II (PSII) efficiency was determined by measuring the 'dark-adapted' ratio of variable to maximum chlorophyll fluorescence, F(v)/ F(m). Both photosystems were more susceptible to photoinhibition at 4 degrees C than at 20 degrees C. However, the degree of photoinhibition was not less in the wild type than in the NDH-deficient plants. To evaluate the efficiency of P700 oxidation in far-red light, a saturation constant, K(s), was determined, representing the far-red irradiance at which half of the maximum P700 absorbance change was reached. In photoinhibited leaves, a decrease in the efficiency of P700 oxidation (increase in K(s)) was observed. The increase in K(s) was more pronounced at 4 degrees C than at 20 degrees C, but not significantly different between wild-type and DeltandhCKJ plants. Re-reduction kinetics of oxidised P700 in the dark were accelerated to a similar extent in photoinhibited samples of both genotypes and at the two temperatures tested. The data indicate that NDH-mediated cyclic electron flow does not protect PSI against short-term light stress. It is proposed that the observed increase in K(s) represents a protective mechanism that is based on accelerated charge recombination in PSI and facilitates thermal dissipation of excessive light energy.     

Modulating Effect of Far-Red Light on Activities of Alternative Electron Transport Pathways Related to Photosystem I

Barley (Hordeum vulgare L.) leaves were irradiated with far-red (FR) light of various intensities after different periods of dark adaptation in order to investigate activities of alternative electron transport pathways related to photosystem I (PSI). Photooxidation of P700, the primary electron donor of PSI, was saturated at FR light intensity of 0.15 μmol quanta/(m² s). As the photon flux density was raised in this range, the slow and middle components in the kinetics of P700⁺ dark reduction increased, whereas the fast component remained indiscernible. The amplitudes of the slow and middle components diminished upon further increase of FR photon flux density in the range 0.15–0.35 μmol quanta/(m² s) and remained constant at higher intensities. The fast component of P700⁺ reduction was only detected after FR irradiation with intensities above 0.15 μmol quanta/(m² s); the light-response curve for this component was clearly sigmoid. In dark-adapted barley leaves, three stages were distinguished in the kinetics of P700 photooxidation, with the steady state for P700⁺ achieved within about 3 min. In leaves predarkened for a short time, the onset of FR irradiation produced a very rapid photooxidation of P700. As the duration of dark exposure was prolonged, the amplitude of the first peak in the kinetic curve of photoinduced P700 photooxidation was diminished and the time for attaining the steady-state oxidation level was shortened. After a brief dark adaptation of leaves, ferredoxin-dependent electron flow did not appreciably contributed to the kinetics of P700⁺ dark reduction, whereas the components related to electron donation from stromal reductants were strongly retarded. It is concluded that FR light irradiation, selectively exciting PSI, suffices to modulate activities of alternative electron transport routes; this modulation reflects the depletion of stromal reductants due to continuous efflux of electrons from PSI to oxygen under the action of FR light. __________ Translated from Fiziologiya Rastenii, Vol. 52, No. 6, 2005, pp. 805–813. Original Russian Text Copyright ? 2005 by Egorova, Drozdova, Bukhov.     

The properties of cytochrome f and P700 in chloroplasts suspended in fluid media at sub-zero temperatures.

1. The properties of P700 and cytochrome f have been studied at sub-zero temperatures in chloroplasts suspended in a medium containing 50% (v/v) ethylene glycol. The dark reduction of these components after a period of illumination provided information about the rate-limiting step of photosynthetic electron transport under these conditions. 2. The oxidation of P700 on illumination in the presence of methyl viologen and its subsequent dark reduction can be observed at -35 degrees C. This cycle of reactions could be repeated many times. The rate of reduction was increased by NH4Cl and reduction was inhibited by 3(3,4-dichlorophenyl)-1,1-dimethylurea. 3. The oxidation and reduction of cytochrome f could also be observed under similar conditions. The activation energies for the reduction of cytochrome f and P700 are similar (about 75 kJ mol-1) and the reduction of cytochrome f is also inhibited by dichlorophenyldimethylurea and stimulated by NH4Cl. 4. The reduction of both cytochrome f and P700 seemed to follow first-order kinetics, but the t1/2 for the redcution of the cytochrome was at least three times that for the reduction of P700 at the same temperature. It was concluded that the results were only compatible with a model in which the main pathway of electrons from plastoquinone to P700 involved cytochrome f if the equilibrium constant between the cytochrome and P700 was very much less than that expected from their redox potentials.     

Pool Size of Electrons That Can Be Donated to P700+ As Determined in Intact Leaves: Donation to P700+ from Stromal Components Via the Intersystem Chain

A method for estimation of the functional pool size of electronsthat can be donated to P700⁺ after the illumination of intactleaves with actinic light is described. The complementary areasbetween the stationary level of P700⁺ attained by irradiationwith far-red light and the oxidation curves of P700 by far-redlight after a 50-ms multiple-turnover light (MT-area), and afterthe illumination of actinic light (AL-area) were determined.Since the MT-area represents the pool size of electrons in theintersystem chain, the ratio of the AL-area to the MT-area allowsus to estimate the pooi size of electrons stored during actinicillumination that can be donated to P700⁺ The ratio of the AL-areato the MT-area was determined for intact leaves of several C₃plants, and it was found to increase to a value of two or threewith increases in the intensity of actinic light, and it wasfurther increased by anaerobiosis. The pool size of electronsthat can be donated to P700⁺ from the stroma was estimated tobe in the range of 12 and 28 electrons per P700 under aerobicconditions. These results indicate that stromal components donateelectrons to P700⁺ through the intersystem chain in chloroplastsof C₃ plants. (Received July 3, 1992; Accepted September 29, 1992)     

Light-induced absorbance changes due to Photosystems 1 and 2 in spinach chloroplasts at −50 °C

Absorbance changes in the region 500–565 nm and at 702 nm, brought about by excitation of Photosystems 1 and 2, respectively, were measured in spinach chloroplasts at ?50 °C. Either dark-adapted chloroplasts were used or chloroplasts preilluminated with a number of short saturating flashes just before cooling.Both photosystems were found to cause a light-induced increase of absorbance at 518 nm (due to “P518”). The System 1-induced change was not affected by preillumination. It decayed within 1 s in the dark and showed similar kinetics as P700. Experiments in the presence of external electron acceptors (methylviologen or Fe(CN)₆^3?) suggested that P518 was not affected by the redox state of the primary electron acceptor of System 1. The absorbance increase at 518 nm due to System 2 decayed in the dark with a half-time of several min. The kinetics were similar to those of C-550, the presumed indicator of the primary electron acceptor of System 2. After two flashes preillumination the changes due to P518 and C-550 were reduced by about 40%, and a relatively slow, System 2-induced oxidation of cytochrome b₅₅₉ occurred which proceeded at a similar rate as the increase in yield of chlorophyll a fluorescence. The results indicate that at ?50°C two different photoreactions of System 2 occur. One consists of a photoreduction of the primary electron acceptor associated with C-550, accompanied by the oxidation of an unknown electron donor; the other is less efficient and results in the photooxidation of cytochrome b₅₅₉.     

Effects of Dark Adaptation on Light-Induced Electron Transport through Photosystem I in the Cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803

Kinetics of the redox reactions in the reaction center (P700) of photosystem I (PSI) of the cyanobacterium Synechocystis sp. PCC 6803 have been studied by EPR spectroscopy. The redox kinetics were recorded based on accumulation of the EPRI signal when the final signal was the sum of individual signals produced in response to illumination of the cells. After prolonged (more than 3 sec) dark intervals between illuminations, the kinetic curve of the EPR signal from P700+ was multiphasic. After a sharp increase in the signal amplitude at the beginning of illumination (phase I), the amplitude rapidly (for 0.1-0.2 sec) decreased (phase II). Then the signal amplitude gradually increased (phase III) until the steady rate of electron transfer was established. With short-term (1 sec) dark intervals between the flashes and also in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), the kinetics of the light-induced increase in the EPR signal from P700+ were monophasic. Inhibition with iodoacetamide of electron transport on the acceptor side of PSI under anaerobic conditions or an increase in the amount of respiration substrates on addition of glucose into a suspension of DCMU-treated wild-type cells increased the level of P700 reduction in phase III. The findings suggest that the kinetic curve of the EPR signal from P700+ is determined by both the electron entrance onto P700+ on the donor side of PSI and activity of electron acceptors of PSI.     

Multi-Phasic Kinetics of P700+ Dark Re-Reduction in Nicotiana Tabacum

Reduction kinetics of P700⁺ after far-red radiation (FR)-induced oxidation in intact tobacco leaves was examined by analysing the post-irradiation relaxation of 810–830 nm absorbance difference. The reduction curve could be de-convoluted distinctively into two or three exponential decaying components, depending on the FR irradiance, the treating and measuring temperatures, and the extent of dark adaptation. The multi-phasic kinetics of P700⁺ re-reduction upon the turning off of FR irradiation is related to the heterogeneity of electron transport around photosystem 1 in thylakoid membranes.     

The use of light-induced absorbance changes at 820 nm to monitor the oxidation state of P-700 in leaves

Abstract The use of the light-induced absorbance change at 820 nm (ΔA 820) to monitor the oxidation and reduction of P-700 in irradiated leaves is examined. Results obtained from leaves irradiated with a range of wavelengths of light, poisoned with DCMU, or lacking PS I, are consistent with the proposition that the light-induced ΔA 820 can be used to monitor P-700 oxidation in leaves.